automatic petrol station prices

These new automatic petrol stations offer real low cost fuel to card holders- However the
delivery prices are few and fixed.
I am concerned about the procedure if I programme, say, 40 euros of diesel and my car
only takes 30 euros worth, do you make a claim or is your card automatically adjusted with
a credit. TIA for putting my mind at rest

Your card gets refunded.

Automatic refund of the rest.
I always fill up so put Lleno into terminal and fill.
Card initially shows 100€ but only charged actual.
(N.B. if you use a UK card they then deduct 2 conversion charges - € to pound and back for the 100€ and then for the refunded ammount, so I always use my Spanish credit card - no problem )

Thank you both for your professional help